Utils: Fix persisting of QRect

And pass explicitly a QVariant to the writer to avoid
unexpected behavior when trying to store the geometry
of a widget.
This resulted in a warning for any session where the
user had used the debugger at least once.

Change-Id: I5cb2352ab29ff7d2a64c84ffcdf040efe6d04b94
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/watchhandler.cpp

@@ -297,7 +297,7 @@ public:
 
     void saveGeometry()
     {
-        SessionManager::setValue("DebuggerSeparateWidgetGeometry", geometry());
+        SessionManager::setValue("DebuggerSeparateWidgetGeometry", QVariant(geometry()));
     }
 
     ~SeparatedView() override